When can I mix my new birds with older birds

We have 6 15 month old birds and 3 3 month old Americaunas . I put them all together in main run and house. I also put their smaller box inside main house so there would be no contention for roosts yet. Older birds are beating the daylights out of younger birds. The young birds started staying inside all day to avoid gettng beat on, missing water and feeding opportunities. I finally re-seperated them. How long should I wait?

I wont combine my flock with my adolescents until they are all roughly the same size.. Also they should be housed where they can see eachother so they arent complete strangers.. Also I integrate in a nuetral area, so the older girls arent so terratorial...